When gasifying carbonaceous fuels, for example, bio fuels or waste-derived fuels, a limited amount of air and/or oxygen and water vapor is generally introduced into a gasification reactor to generate product gas, the major components of which are carbon monoxide CO, hydrogen H2, and hydrocarbons CxHy. The product gas being discharged from the gasification reactor may transfer with it ash particles and residual carbon, which must be separated by a particle separator, for example, by a filter, before the product gas is further utilized. The aim is to optimize the efficiency of the gasification system by maximizing the carbon conversion of the fuel, in other words, the residual carbon content of the ash to be discharged from the apparatus will be as low as possible.
U.S. Pat. No. 4,347,064 discloses an apparatus, in which the particles separated from the product gas of a first fluidized bed gasifier are conducted to a second fluidized bed gasifier, in which residual carbon contained in the ash is gasified and a portion of the generated product gas is conducted back to the first fluidized bed gasifier. Finnish Patent No. 110266 discloses a method, in which residual carbon contained in the ash, which is separated from the product gas of a fluidized bed gasifier is combusted in a fluidized bed combustion plant, and the generated oxygen-containing flue gas is conducted to act as secondary gasification gas of the fluidized bed gasifier.
U.S. Pat. No. 3,454,383 discloses a method, in which residual carbon separated by a particle separator from the product gas is combusted together with fine fuel coal in a cyclone combustor with theoretical to slightly excess air, whereby coal combusts completely and melt slag and very hot carbon dioxide-containing exhaust gas are generated. The hot exhaust gas is conducted through a narrow throat at the bottom of the gasification reactor to a widening lower portion of the reactor, where it fluidizes the coarse carbon particles introduced into the lower portion, layered according to the particle size. Thereby, the carbon gasifies gradually, and the smallest particles flow with the product gas to the product gas channel connected to the upper portion of the reactor. The difficulty with the method disclosed in the patent is that the high temperatures prevailing at the lower portion of the reactor easily lead to agglomeration of the fuel, especially, when gasifying bio fuels (e.g., wood, straw, agro fuels, etc.), or waste-derived fuels.